The problem for Facebook is that these people can't buy anything online, because they don't have a bank account. If Libra launches successfully and remains stable, those same places selling X MB of data for cash, could easily start selling Libra too.
That's what so many people here are missing. Facebook isn't targeting this at people who have credit cards, Paypal accounts, and know what Bitcoin is, they're targeting the day laborer in a rural, 3rd world country, who might be enticed to part with some of their spending money for some Chinese plastic trinket that they never could have bought before, because none of the few stores in a 100 mile radius stock stuff like that.